BIOL126 Lecture Notes - Lecture 8: Allantois, Neural Groove, Foregut
8 Jun 2018
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Embryological Development
• Three steps
o Divide (cleavage stage)
• 24hrs after fertilisation
• A few days later -> morula
• 1 Zygote - > 16 blastomeres
• 3 days after fertilisation -> morula
• End of cleavage stage
o Differentiation
• Morula -> blastocyte (sphere of fluid)
▪ Blastocyte: large flat outer layer of trophoblast cells and cluster of inner cell
mass
▪ Inner cell mass will turn into embryo
▪ Trophoblast cells will form placenta
o Develop
• Embryo -> foetus
• Summarise the major stages of development of the embryo and foetus
o Stages of development
• Pre-embryonic
▪ Weeks 1-3
▪ From fertilisation to gastrulation (development of the trilaminar disc)
• Embryonic
▪ Weeks 4-8
▪ Rapid differentiation and formation of the major organ systems
• Foetal
▪ Weeks 9-41
▪ Growth
o Fertilisation
• Fusion of the ovum and sperm to form the zygote
• Consequences
▪ Restoration of the diploid number of chromosomes (23+23 = 46)
▪ Determination of the sex of the zygote
▪ Activation of the ovum
• Changes in chemistry and structure prevent entry of a second sperm
▪ Initiation of cleavage
o Cleavage and the development of the blastocyst
• Cleavage
▪ A fractioning process
▪ Rapid mitotic divisions - blastomeres
▪ 12-16 cell stage - 'morula'
▪ Zona pellucida - covering
• Holds cells together
• Prevents attachment to uterine tube wall
• Formation of the blastocyst
▪ Glandular secretions from the uterine tube enter the morula, pushing some
cells to one pole - inner cell mass
▪ Outer cells are pushed flat against the zona pellucida - outer cell mass
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o Implantation
• Adherence and attachment of the blastocyst to the endometrium (uterine wall)
• Commences 7-8 days after fertilisation with disappearance of the zone pellucida
• Completed around 14 days post fertilisation
• Trophoblast buries the blastocyst into the uterine wall
• Describe the process of gastrulation
o Embryonic disc and germ layers
• Formation of the bilaminar disc
▪ At around day 8, the inner cell mass differentiates into two layers
• Cuboidal cells = endoderm
• Columnar cells = ectoderm
▪ Appearance at day 14
• Formation of the trilaminar disc
find more resources at oneclass.com
find more resources at oneclass.com
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Inner cell mass will turn into embryo: trophoblast cells will form placenta, develop, embryo -> foetus. Summarise the major stages of development of the embryo and foetus: stages of development, pre-embryonic, weeks 1-3. From fertilisation to gastrulation (development of the trilaminar disc: embryonic, weeks 4-8, rapid differentiation and formation of the major organ systems. Initiation of cleavage: cleavage and the development of the blastocyst, cleavage, a fractioning process, rapid mitotic divisions - blastomeres, 12-16 cell stage - "morula" Zona pellucida - covering: holds cells together, prevents attachment to uterine tube wall. Formation of the blastocyst: glandular secretions from the uterine tube enter the morula, pushing some cells to one pole - inner cell mass, outer cells are pushed flat against the zona pellucida - outer cell mass. Formation of the bilaminar disc: at around day 8, the inner cell mass differentiates into two layers, cuboidal cells = endoderm, columnar cells = ectoderm, appearance at day 14.
